Historically, building air conditioning systems have employed several schemes of capacity control.
One such scheme is to supply air at a constant volume to the conditioned space. As the building air conditioning load varies, the temperature of the air supply is varied. While some systems provide precise humidity control via reheat, others provide no humidity control. The system is relatively low in cost and simple to design. Where humidity control via reheat is employed, operating costs may be expensive.
Another system which is in use is known as a variable air volume system. In this system, the volume of the air is infinitely and continuously varied on a room-by-room or zone-by-zone basis usually while the supply air temperature remains constant. This requires that the volume of building supply air from its source will vary continuously in accordance with the total needs of the area served by the source. It will be evident that such systems can also provide zone temperature contol flexibility. Another advantage over the constant volume system that may be achieved in variable air volume systems is an energy savings resulting from reduced air flow during periods of reduced air conditioning demand. A major disadvantage of the variable air volume system is its initial cost. In many buildings a variable air volume system cannot be justified for one or more of several reasons. The absence of a need for individual zone temperature control may be one. The initial cost may be another. The complexity of designing the system and selecting the components may be still another.